Streetsblog received this tip last night:

I was riding North on the Greenway tonight at about 9:15, and after clearing 60th street and entering the stretch under the elevated
Miller Highway a southbound cyclists yelled at me, "Watch out!
There’s a pile of rocks across the bike path ahead."

I slowed down and carefully rode to 67th Street where what I saw was
quite a shock.

A pile of ornamental rock had been taken from the the Riverside Park
South border of the greenway and piled across the bike path.
The pile was about 1 foot high in some places, and went across the entire bike path, and had already been scattered by what looked like
an earlier collision. It was clearly intended to cause a serious 
accident. The bike path is largely unlit in this stretch, and even
with a bike light, the pile was hard to see.

Together with 4 other cyclists who stopped to help we cleared the
obstruction.

I also called 311 on the scene who connected me to 911 who promised to
dispatch a patrol car that never came after about 15 minutes. I
have a card from one of the volunteers and one of the others also
promised to email me with his contact info.

In addition to you, I was going to follow up w the Riverside Parks
Commissioner, the Riverside Parks Fund, and see if I can get my case
number from 311. Who else should i/we contact?

On a positive note I can report that even on this 32-degree evening
the bike path was well traveled, and we had plenty of volunteers who
quickly cleared the obstruction. I took this as a recreational ride
to decompress from a meeting that ended at about 8p from Lincoln
Center, riding down the Greenway (which was free of obstruction at the time) and then headed back up from the Battery. I would estimate
that I encountered about 50 cyclists, a healthy increase over what it
usually is on a relatively cold winter night. And the other bit of
good news is that there was not a single one without lights — which
is real progress.

The bad news is that there is a seriously dangerous vandal out there
out to cause harm to cyclists. And how this happened unobserved given
all the use of the path that I saw is also very disconcerting.